Sag Theory (Paint Sag Equation)
Flow marks caused by gravity-driven wet coatings are called sags. The sag model is shown in Figure 3-9-1. The pulling force (i.e. the shear stress t x ) that causes the sagging of the thin layer at the position x from the surface in the coating film:
Obviously, Newtonian coatings have poor sag resistance, and the sagging becomes more serious as the film thickness increases, but in fact, the coatings are all pseudoplastic, and the solvent volatilization during the flash-drying process reduces the temperature and increases the concentration. , causing the viscosity to rise; at the same time, it also makes the coating film thinner. All these factors can suppress the sag. However, the viscosity, as the sag resistance, is always the main factor that should be considered in the formulation design. Usually, adding a small amount of rheological additives to the In Newtonian coatings, make the viscosity at low shear rate reach tens to hundreds of neu-seconds or give the coating a yield value that resists sagging.
Since the effect of viscosity on leveling and sagging is opposite, changing the viscosity can only adjust the leveling and sagging to an appropriate level. To effectively solve the contradiction between leveling and sagging, only by using the angular denaturation to give the system an appropriate yield value, can the extreme leveling and sag resistance be obtained.
Sag Precautions
According to the sag equation, the main factors that cause sagging are the thickness of the coating film and the viscosity of the coating. Among them, the viscosity of the paint is also affected by temperature, solvent volatilization and denaturation of the paint angle. The root causes and countermeasures are described below.
1 The reason why the sprayed wet film is too thick
(1) Repeated spraying multiple times;
(2) The distance between the Spray Gun and the workpiece is too close;
(3) One spray is too thick, such as the Spray Gun moves too slowly or stops.
Solution:
(1) During wet-on-wet construction, there should be sufficient flash-off time between two sprays, and the flash-off time for thin coatings should be 3 to 5 minutes. As the number of sprays and the total thickness of the wet film increase, the flash-off time should be extended to 5 minutes at the end. ~8min;
(2) Ensure that the distance between the Spray Gun and the workpiece is vertical and the distance is constant;
(3) The moving speed of the Spray Gun should be uniform, avoid stopping, and avoid shooting directly at the workpiece and turning back.
2. Coating viscosity is too low
Strictly control the dilution ratio and dilute to the specified construction viscosity according to the type of paint.
3. The solvent evaporates too slowly
Such as ① improper selection of diluent; ② construction environment temperature is too low. The solution is to ① use a special thinner; ② increase the ambient temperature and enhance ventilation.
4. Spray on smooth old paint film
The wettability of the paint on the surface is poor, and it is easy to cause slump. Sand and roughen the old paint film in advance.
